Earnings Per Common Share Offerings (Details) - USD ($) $ / shares in Units, $ in Millions		12 Months Ended
	Dec. 12, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Weighted average shares—Basic		61,579,000	57,324,000	56,734,000
Class A Common Stock [Member]
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Weighted average shares—Basic			131,148
Class A Common Stock [Member] | Common Stock [Member]
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Stock issued during period, shares	4,000,000	4,000,000
Shares Issued, Price Per Share	$ 75.00
Net proceeds from issuance of common stock	$ 285.8